Electroplating has many applications. One very important developing application is in plating copper onto semiconductor wafers to form conductive copper lines for “wiring” individual devices of the integrated circuit. Often this plating process serves as a step in the damascene fabrication procedure.
A continuing issue in modern VLSI wafer electroplate processing is quality of the deposited metal film. Given that metal line widths reach into the deep sub-micron range and given that the damascene trenches often have very high aspect ratios, electroplated films must be exceedingly homogeneous (chemically and physically). They must have uniform thickness over the face of a wafer and must have consistent quality across numerous batches.
Some wafer processing apparatus are designed to provide the necessary uniformity. One example is the SABRE™ clamshell electroplating apparatus available from Novellus Systems, Inc. of San Jose, Calif. and described in U.S. Pat. Nos. 6,156,167 and 6,139,712, which are herein incorporated by reference in their entireties. The clamshell apparatus provides many advantages for wafer throughput and uniformity; most notably, wafer back-side protection from contamination during electroplating, wafer rotation during the electroplating process, and a relatively small footprint for wafer delivery to the electroplating bath (vertical immersion path).
Modifications to the “clamshell” and its associated plating environment for improved wafer uniformity and quality have been described in U.S. Pat. Nos. 6,074,544, 6,110,346, 6,162,344, and 6,159,354 which are herein incorporated by reference in their entireties. The described modifications relate to methods for using variable currents, improved mass transfer, electric potential shaping, and the like.
Although plating quality continues to improve with developments such as those described in the above patents, there is a continuing need for improved uniformity and consequent higher plating quality. The local environment seen by the wafer during plating drives the deposited layer uniformity. Plating solution flow patterns, bubbles, electric field shape, and the like all affect the quality of the deposited metal film. One issue of particular importance is the electrolyte velocity distribution across the wafer surface, particularly the distribution of the component normal to the wafer surface. For many combinations of apparatus and operating conditions, the electrolyte velocity varies significantly in the radial direction across the wafer surface. Because the plating rate and quality is a function of local velocity, this condition causes uneven plating thickness and/or quality.
With the deleterious effects of uneven plating solution flow patterns in mind, tighter control of certain design parameters should be realized. What is needed therefore is improved technology for controlling plating solution flow dynamics with respect to the wafer surface during electroplating.